"The Thought-Monster" is a short story written by Amelia Reynolds Long. It was published in the March 1930 issue of Weird Tales.
A small town experiences a rash of mysterious deaths. Victims turn up dead with looks of horror on their faces. The culprit is dubbed "the terror" by the locals. A visiting policeman from New York City, Detective James Gibson, thinks it's just an escaped lunatic, and leads an Angry Mob to catch him. He turns up later, having been transformed into a gibbering idiot.
The terrified townsfolk turn to a "psychic investigator" named Michael Cummings, who is convinced that the killings are the work of an invisible supernatural force. Specifically, he thinks it has something to do with a scientist named Dr. Julian Walgate who lives outside of town, and he encourages the townspeople to discourage "the terror" from attacking by rigging up violet lights around their homes after dark, because (according to him anyway) violet light drives away supernatural beings.
Surprisingly, it works. When the lights are installed, the killings taper off and eventually stop... only to start up again after the creature apparently develops an immunity to the light. Cummings and coroner Dr. Bradley then go to confront Walgate after receiving a mysterious phone call from him, only to discover his journal detailing his experiments in the materialization of thought...
The Thought-Monster was adapted into the 1958 film Fiend Without a Face. It was directed by Arthur Crabtree with a screenplay by Herbert J. Leder, who relocated the story to Canada (The Thought Monster never really reveals where it takes place). Leder also changed Michael Cummings into an Air Force Major named Jeff Cummings and added a subplot about the USAF conducting long-range radar tests, among other changes and expansions.
A very well-produced audio version read by Edward E. French can be enjoyed here.
The Trope Monster:
- Agent Mulder: "Psychic investigator" Michael Cummings, who believes in supernatural phenomena apparently purely out of habit.
- Agent Scully: Dr. Bradley. He's skeptical once the psychic introduces his plan to keep "the terror" at bay using violet lights. However, he is forced to admit that Cummings' plan worked after a month passes with no deaths.
- Angry Mob: Gibson leads one to catch his imaginary escaped lunatic.
- Antagonist Title: The "terror" menacing the town is a creature of thought. Therefore, a thought monster.
- Apocalyptic Log: Dr. Walgate's journal.
- Brain Food: The monster consumes its victims' minds.
- Crazy Enough to Work: Cummings' plan to repel the invisible menace using violet light. It works for a while. Soon the killings start upon again, however, when the thought monster develops a resistance to the lights. Eventually, inspired by Cummings, Dr. Walgate is able to kill the invisible menace using violet lights of an intensity that the creature can't withstand, although it costs his his sanity.
- Dead Foot Leadfoot: After the terror grows strong enough to resist the power of the violet lights, it kills a carload of people. The corpse-filled vehicle comes barrelling into town with the driver still gripping the steering wheel and pressing his foot on the accelerator.
- Death of a Child: Two of the creature's victims include two little boys.
- Doing In the Wizard: Other than simply doing it For Science!, this appears to have been the original motivation behind Walgate's work. He wanted to prove that so-called psychic phenomena are the result of a person's thoughts made physical and having an impact on the surrounding environment, especially providing a scientific explanation for "table-tipping" and other parlor trucks.
- Driven to Madness: Gibson, following his encounter with "the terror." This is also what happens to Dr. Walgate at the end. See Heroic Sacrifice.
- Eye Remember: Cummings uses the theory that the last thing a person sees upon dying is imprinted into their eyes, asking coroner Dr. Bradley if he can photograph the eyes of the first person who dies after he arrives, a farmer. It doesn't work because the creature is invisible. Something Cummings himself admits:Cummings: There is one alternative. Perhaps there was nothing for the dying man to see.
- For Science!: This seems to be Dr. Walgate's entire motivation. There is a bit of Doing In the Wizard behind his work, but past a certain point, he goes into full-blown Mad Scientist mode and becomes obsessed not with proving that psychic phenomena are caused by people's thoughts given physical form, but with creating a being made of living thought.
- Gone Horribly Wrong: Or Gone Horribly Right, depending on your interpreation. Walgate wanted to create a being of living thought, but he had no idea it would eat people's minds, either killing them or driving them insane.
- Heroic Sacrifice: Dr. Walgate uses himself as bait to lure the "terror" into a lead-lined room where he traps it and kills it using violet light, and is found by Cummings and Bradley "a gibbering idiot."
- Invisible Monsters: "The terror" is completely invisible.
- It's Quiet Too Quiet: When visiting the Walgate residence, Cummings and Bradley notice that no birds sing in the surrounding forest and that the house itself is deathly quiet.
- Interrogating the Dead: Cummings tries to use the old "the last thing a person sees is imprinted into their eyes method" with the dead body of a farmer who dies after be arrives. Walgate himself also disinters the body of the mayor to conduct his own investigation, fearing his creation is responsible for the deaths.
- Know-Nothing Know-It-All: Detective Gibson. For all the talk about the string of solved cases he has under his belt back in New York, he isn't that impressive and doesn't come across as very smart.
- Mad Scientist: Walgate, although not necessarily the evil variety.
- Occult Detective: Michael Cummings, psychic investigator extraordinaire.
- Offscreen Moment of Awesome: The story ends with "the terror" having already been killed. The reader is only told of the aftermath.
- Police Are Useless: Detective Gibson is convinced that the killer is a deformed convict whose very appearance frightens his victims to death (!) and he pursues this (highly improbable) lead to the exclusion of everything else.
- Start to Corpse: Retired farmer Wellton Grim is dead in the first sentence of the story.
- Title Drop: In his journal, Walgate writes, "I have let a thought-monster loose upon the community!"
- Trampled Underfoot: Three people get trampled by the mob running in fear from the building when the terror strikes a town meeting.
- Unwitting Instigator of Doom: Walgate's servant Mrs. Jenson unintentionally lets the thought monster loose when she goes into the laboratory "in a fervor of housecleaning".